Apparatuses for compression of hot dross of non-ferrous metal, preferably aluminum, alloys thereof, magnesium, zinc dross, comprising dross compressing head with ribbed or non-ribbed surfaces and skim pan with or without ribbed surfaces, as well as having one or more through drainage holes in bottom are known in the prior art.
Known is an apparatus for compression of hot dross of non-ferrous metal, preferably aluminum, alloys thereof, magnesium, zinc dross, comprising frame with dross compressing externally ribbed head of semispherical or elongated semispherical shape; skim pan with one or more through drainage holes in bottom, wherein the skim pan is located on the top of sow mould, which collects drained from dross metal. (U.S. Pat. No. 5,397,104, March 1995. Roth D.).
Known is the method (U.S. Pat. No. 4,386,956, June 1983, Roth D.) for recovery of free aluminum metal from a metal bearing dross containing free aluminum metal and oxides of said metal, said method comprising the steps of:                (a) collecting the dross in a skim pan having at least one trough with at least one downwardly and inwardly inclined side wall with metal flow passages through the wall;        (b) collecting the free aluminum metal which decants through the passages;        (c) mechanically compacting the dross by applying a compressive force on the dross and against the inclined wall above a threshold pressure to pool free aluminum metal within a dross, effect migration of the free aluminum metal toward a boundary surface of the compacted dross, decant a portion of said free aluminium through the passage and inhibiting the oxygen supply to the free aluminum metal in the dross; and        (d) cooling the compacted dross and free aluminum metal to solidify the free aluminum metal; such that thermite reaction of free aluminum metal and oxygen in the dross is diminished by the combination of compacting above the threshold pressure and cooling of the metal.        
Known is the method (U.S. Pat. No. 4,565,572, January 1986 van Linden at al.) for the recovery of free aluminum metal from hot aluminum dross which forms on the surface of molten aluminum during aluminum melting operations, said dross containing free aluminum metal and a solids network, said method comprising the steps of:
(a) charging the hot dross from the aluminum melting operation into a receptacle having at least one inclined sidewall and at least one passageway;
(b) mechanically compacting the dross with a heated ram, said compacting applying a compressive force on the dross towards the inclined sidewall such that a substantial portion of the free aluminum separates from the dross and passes through the bottom passageway;
(c) maintaining the temperature of the hot dross during its collection in the receptacle and subsequent mechanical compaction so as to enhance the removal of free aluminum therefrom; and
(d) collecting the free aluminum metal which passes through the bottom passageway.
A drawback of the aforementioned method is that due to the preliminary heating of the head, and other conditions being equal, the dross cooling cycle time increases, and hence, as a result of longer oxidation time of aluminum, the metal recovery decreases.
A drawback of the aforementioned methods and apparatus is that the amount of metal, drained from hot dross, the so called in-house metal recovery, is usually relatively lower (from 3 to 20%), than the metal content remaining in dross skull after freezing (from 40 to 70%). Further secondary recovery of metal from frozen dross is as a rule accompanied with its supplemental mechanical and/or thermal treatment (melting), which results in additional metal loss.
Known is the prototype for recovery of non-ferrous metal from hot dross, comprising a frame with dross compressing head, a sow mould for collection of drained metal from dross and a skim pan, provided with one or more through drainage holes in a bottom part on said sow mould, characterized in that provided with at least one through hole with connection for switching on vacuum through said sow mould wall, sealing located in the gap between said skim pan and sow mould.
A drawback of the aforementioned prototype is that the location of the through hole with a connection for switching on a vacuum through said sow mould wall, in the case of hot dross with a high aluminium content (more than 55%), results in an overlapping/blocking of the aforementioned hole by drained metal in the sow mould when completely full. It does not allow more metal to drain into the sow mould.